Reflectors are used in EUV lithographic apparatus because EUV radiation is strongly absorbed in most materials and thus transmissive lenses are not practical for EUV lithographic apparatus. When a reflector is used to reflect EUV radiation some of the EUV radiation is absorbed by the reflector (reflection of the EUV radiation is not 100% efficient). Absorption of the EUV radiation by a reflector causes heating of the reflector, and this will in turn cause distortion of the reflector. Distortion of the reflector is undesirable because it will introduce aberrations into the EUV radiation beam. If the reflector is a diffraction grating an undesirable effect of the heating may be to distort the grating such that diffraction orders are no longer generated in a desired manner by the diffraction grating.
It may be desirable to obviate or mitigate at least one problem associated with known reflectors.